There are a lot of viruses out there that embed themselves in a host's cells/DNA and can lay dormant for years, or even decades, before becoming activated by the right conditions.

I've always assumed that everyone was infected by the virus, which is now laying dormant in their bodies until being activated by their death. The reason bites, and even just scratches, kill are because the "active" version of the virus is introduced and activates the dormant version of the virus already present in the victim's body. It also explains how someone who has just turned, and wouldn't have built up the kind of bacteria to kill someone, can still kill someone from a single bite.

I don't know if this is canon or not, but it makes sense to me. In the end, it's just an excuse to set up the rules/conditions of this world. How we get there isn't really all that important.

*EDIT: And that's why you read a thread all the way to the end. DEADinTHEhead beat me to it.

I think of it this way: everyone is infected, but until a person dies the virus cant invade the "blood/brain barrier" and reproduce because of the body's immune system, so the virus stays in check. But in the case of a bite, the virus has already crossed the barrier in the walker doing the biting, where it reproduced unchecked, and when the walker bites someone, massive amounts of virus are transfered into the victim's bloodstream, on top of what was already there. This sets the immune system into overdrive, hence the extreme fever a bite victim experiences. The massive immune system response to the virus kills the victim, and then the virus moves through the unprotected "blood/brain barrier" and reproduces, starting the cycle again.

That isn't what Ben said. He said that it isn't just the bite that makes people turn, as that is what they previously believed. He s… moreaid that they come back no matter how they die as long as the brain isn't destroyed, but the bite speeds up the process.

Everyone has the virus inside them but it only activates once the person's dead. The bite actually does not contain the "Walker Virus" but the saliva of those walkers contains many deadly pathogens that humans can't fight against, thus killing them in a matter of hours. Then the "Walker Virus" activates and brings them back as mindless flesh-eating corpses.

The bite is just a sure-kill method. Well, that's how I see it at least.